1. Field of the Invention
This invention relates to curable organosiloxane compositions. More particularly, this invention relates to organosiloxane gels having a non-reactive silicone fluid therein, wherein the improved gel has less weight loss and improved heat stability, with no appreciable bleed.
2. Background Information
In many instances gels used as coating, potting, and encapsulating materials must maintain adhesion to electronic and electrical components and printed circuit boards in addition to the electrical connectors and conductors that pass through the coating or encapsulating material. Organosiloxane gels electronic applications are typically prepared from compositions containing high molecular weight polyorganosiloxanes. These materials that form the gels are expensive, and many applications would benifit from a gel that has the desired properties but at a reduced cost. In addition, it would be desirable to reduce the amount of the gel that evaporates and to improve the stability of the gel when in is exposed to high temperatures.
The inventors have discovered that large amounts of a non-functional silicone fluid can be added to a silicone gel without the fluid "bleeding out". The non-functional silicone fluid reduces the cost of the gel. The inventors have surprisingly found that adding a non-functional silicone fluid reduces the weight loss of the gel over time, and improves the heat aging properties of the gel as well. The gels may be used as an encapsulant or pottant to protect electronic circuit boards, or a material useful for dampening vibrations.
U.S. Pat. No. 5,145,933, which issued to Grisoni et al. on Sep. 8, 1992 describes organosiloxane gel forming compositions that cure by a platinum-catalyzed hydrosilation reaction. The curable compositions contain a polyorganosiloxane with vinyl radicals located only on non-terminal silicon atoms and two different types of organohydrogensiloxanes. One is a polydiorganosiloxane (1) containing silicon-bonded hydrogen atoms only at the terminal silicon atoms, and the second polyorganosiloxane (2) contains an average of at least three silicon-bonded hydrogen atoms per molecule, at least a portion of which are located on non-terminal silicon atoms. The percentage of the total silicon bonded hydrogen contributed by polyorganosiloxane (1) is equal to 81.36 minus a factor equal to 3.6 times the molar ratio of all silicon bonded hydrogen atoms in the curable composition, irrespective of source, to all of the alkenyl radicals present in the composition.
U.S. Pat. No. 4,340,709, issued on Jul. 20, 1982 to Jeram et al. relates to organosiloxane compositions that cure by a platinum-catalyzed hydrosilation reaction and contain the two types of organohydrogensiloxanes described in the Grisoni et al. patent. The molar ratio of the vinyl radicals present in the curable polyorganosiloxane to the silicon bonded hydrogen atoms present in the organohydrogensiloxane (1), referred to by Jeram et al. as a hydride coupler, is from 0.75 to 1.5. While Jeram et al teach that the viscosity of the vinyl-terminated curable polyorganosiloxane can be from 100 to 200,000 centipoise (0.1 to 200 Pa.s) the lowest viscosity disclosed in the exemplified compositions (examples 2 and 3) is a mixture of 61.8 parts of a dimethylvinylsiloxy-terminated polydimethylsiloxane with a viscosity of 4,000 cps (4 Pa.s) and 5 parts of a dimethylvinylsiloxy-terminated polydimethylsiloxane with a viscosity of 0.5 Pa.s. In these compositions the hydride coupler contributes 70 and 74 percent, respectively, of the silicon-bonded hydrogen atoms present in the curable compositions, and the compositions contain a reinforcing silica filler. The curable compositions of example 2 and 3 had mixed viscosities of 40 and 85 Pa.s, respectively.
One objective of this invention is to provide curable organosiloxane compositions which contain a silicone fluid that does not "bleed" from the cured composition. The gel may be used as a pottant or encapsulant, or as a material capable of dampening vibrations.